Which of these numbers is composite? ${2,\ 13,\ 61,\ 89,\ 91}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 13, 61, and 89 each have only two factors. The factors of 91 are 1, 7, 13, and 91. Thus, 91 is the composite number.